1. Field of the Invention
The present invention relates to an image forming method for use in an electrophotographic apparatus, such as a copying machine, a facsimile machine, or a printer.
2. Description of the Related Art
With the improvements in electrophotographic recording apparatuses in recent years, the use of so-called on-demand printing has become common. Here coated paper is used, that previously would have been used in commercial printing, such as offset printing, and a digital signal is color printed on demand on glossy coated paper by an electrophotographic process. Because of this, progress has been made in increasing speeds and improving image quality of color copying machines and color printers.
With respect to a higher image quality, with electrostatic latent images becoming finer due to improvements in resolution of the apparatuses, in order to develop an electrostatic latent image faithfully and obtain images of higher image quality, the particle size of toner is becoming smaller and smaller in recent years. In particular, for a full-color copying machines that develop, transfer, and fix digital latent images with chromatic color toners, image quality has improved to a certain degree by adopting toners having small particle sizes whose volume average particle size is in the range of 6 to 8 μm. However, in order to meet the need for yet higher resolutions (improvements in fine line reproducibility, improvements in gradation, etc.), the particle size of toner has to be made even smaller and to be in an appropriate particle size distribution.
Meanwhile, when the volume average particle size of toner exceeds 8 μm, toner with a bulk density exceeding 0.5 g/cm3, when the toner is layered to form an unfixed image, the voids become so large that toner forming the image is readily scattered, which gives rise to the problem of rough images. Conversely, when a bulk density of toner is too low, voids in the toner forming a toner image become smaller, which readily gives rise to blisters.
A blister is a minute void developed on the surface of a toner image portion. The mechanism responsible for the development of a blister is as follows. That is, water within the paper turns into moisture vapor when heated during the process of fixing the toner image. When the paper has low air permeability, the moisture vapor is not released to the outside, and vapor pressure within the paper increases to the extent that the moisture vapor breaks through the toner layer to be released.
In particular, in the case of coated paper having high sheet gloss for use in commercial printing, the surface of paper is coated then smoothened by calendaring to confer the required sheet gloss to the paper. In this instance, the paper is flattened by a pressure applied during the process of calendaring, and the air permeability is thereby reduced. This readily gives rise to blisters during the process of fixing the toner image.
In addition, in a case where such printing coated paper is used as paper stock, the paper size varies with changes of water content in the paper. In the case of a full-color copying machine or printer adopting the indirect dry-type electrophotographic process, water content in the paper changes when the toner is heat-fixed onto the paper. This poses another problem that the outputted paper curls.
With respect to smaller particle sizes, a toner manufacturing method by emulsion aggregation process is proposed as a method of systematically controlling the shape and the surface structure of toner particles (for example, see JP-A Nos. 63-282752 and 6-250439). Because in an emulsion aggregation process, the particularized raw material of the starting material is normally 1 μm or smaller, in theory, it is possible to manufacture toners of small particle sizes efficiently.
However, a toner of a small particle size manufactured by this method is of a spherical shape, and there is a tendency that the bulk density of the toner is increased, which generally gives an adverse effect on the suppression of blisters. In addition, when a toner of a small particle size is used, the toner falls into the formation of the paper. This causes a problem that resulting images are rough.
In order to cope with this problem, there has been proposed a method for suppressing the occurrence irregularities of releasing agent in a hard copy image by supplying a releasing agent, such as silicone oil, to the fixing member and using a toner furnished with a releasing agent function containing wax (for example, see JP-A No. 2003-316172). However, the occurrence of blisters is not attributable to the chemical properties of the toner, and they occur simply due to the presence of voids resulting from the shape of the toner particles. Hence, blisters still occur even when releasing agent is applied to the fixing member or a toner prepared by adding wax to a pulverized toner is used.
In addition, in order to suppress the occurrence of blisters during the process of fixing toner images, regulating the toner amount on a print medium at the image forming apparatus side has been proposed (for example, see JP-A No. 2003-51902). To be more specific, the toner amount on a print medium is controlled according to print medium used and print mode. Further, an optimum toner amount on a print medium for a toner containing particles having a particle size of 16 μm or greater has been proposed (for example, see JP-A No. 2-183270). However, both proposals fail to take into account interaction in the toner, and therefore it cannot be said that they are sufficient image forming frameworks capable of suppressing blisters and improving image quality.